31P NMR presents a unique approach to non-invasive muscle biochemistry in normal and diseased patients. A 10 1/4" clear bore NMR spectrometer affords an opportunity to evaluate muscle biochemistry in the limbs of normal and diseased humans. The principal changes of muscle bioenergetics over the convenient range of exercise are phosphocreatine (PCr) decrease from approximately 6 times the ATP level to one times the APT level with corresponding rises of inorganic phosphate remaining practically constant. PCr/Pi (or PCr/(PCr + Pi)) assesses the biochemical function of the resting and exercised human forearm and calf muscle and evaluates the competency of oxygen delivery to tissue and the capability of mitochondria to mobilize substrates and utilize oxygen for maintaining PCr levels high and the Pi low. A study of the nature of errors in the 31P NMR study of human tissues is here applied to the human leg under resting and exercise conditions in normal controls and in patients having unilateral peripheral vascular disease. The repeatability and reliability of the data from these groups under these conditions will afford a baseline for the use of NMR as a clinical analytical biochemical tool; the leg being the organ most available in a variety of diseased states, and the data will afford a baseline for future applications to other body organs. Thus the protocols involve appropriate resting states, maximum limb exercise feasible for the particular subject, normal or diseased, followed by insertion of the limb into the NMR spectrometer immediately following a quantitative exercise regimen. Categories of recovery times from this procedure are established and their reliability determined. On this data base, NMR reliability will be established by appropriate statistical methods, with correlations with disease categories obtained from other methods. Prediction of the course of peripheral vascular disease and the effectiveness of therapeutic procedures by the NMR technique may ultimately be possible.